In boiling water reactor (BWR) nuclear fuel bundles, fuel spacers are installed at fixed heights relative to a lower tie plate to maintain rod-to-rod spacing between the fuel rods from the top to the bottom of the bundle. That is, the spacers maintain the fuel rods in an ordered array within the fuel bundle, e.g., 9.times.9 or 10.times.10 arrays of fuel rods. A typical nuclear fuel bundle will contain seven or eight spacers held in place by tabs secured to one or more central water rods. Each spacer consists of a banded lattice of cells through which the fuel rods pass and which cells hold the fuel rods by contact between the fuel rod cladding and stops in the body of the cells and springs forming part of or assembled into the cells. These spacer springs impose a lateral force on each fuel rod, for example, in a range of 1.5 to 3.0 pounds for a newly-manufactured fuel bundle. After a full cycle of in-reactor burn-up, the spring forces are anticipated to be somewhat lower.
During manufacture and prior to assembly of the spacers into a fuel bundle, it is common practice to measure the spring forces on all of the springs using a spacer spring force measurement device. Such device is a gauge in the form of a plunger assembly containing a miniature compression load cell whose signal is monitored and interpreted in a data collection system. The gauge plunger assembly is inserted into each cell of the spacer and the load cell is capable of giving a measurement of the cell's spring force accurate to a few tenths of a pound.
The spacer spring force measurement devices used during manufacture of the spacers, however, cannot be made for measurements of spacer spring forces when the spacers are in the bundle or in irradiated fuel bundles due to the harsh radiation environment, thermal temperatures and water pressures that exist in a nuclear reactor core or storage pool. While in-bundle spacer spring measurements are desirable, formidable technical difficulties have remained. For example, spring force measurements for spacers in-bundle must be made with remote handling equipment due to the high radiation fields. The spacer springs are also not directly accessible in either irradiated or non-irradiated fuel bundles and the spring force measurement equipment for irradiated fuel must be designed to be remotely positioned, usually from the top of the bundle. Additionally, transducers such as load cells conventionally cannot withstand the harsh environment present in irradiated fuel bundles in fuel storage pools. Probes used in spring force measurements must also be designed so that they do not damage the spacers or the fuel bundle, e.g., misalignment of the probes can damage the spacers or spacer springs. Further, the accuracy of the spring force measurements must be comparable to that made for newly-manufactured spacers.